A gas sensor such as an oxygen sensor, a NOx sensor or an HC sensor is known in the prior art. This gas sensor is generally provided with a protector for covering and protecting such a leading end portion of a gas sensor element as will be exposed to the object gas such as an exhaust gas. Some protector is given a double structure by considering the responsibility of the sensor, the water resistance of the gas sensor element and so on. The gas sensor having the protector of such double structure is disclosed in JP-A-2002-162377, for example.
In this gas sensor, the protective tube (or protector) is composed of an outer protective tube (or outer cover portion) and an inner protective tube (or inner cover portion). Of these, the outer protective tube is welded and fixed at its trailing end portion to the outer circumference of the leading end edge of the metal shell. On the other hand, the inner protective tube is welded and fixed on the outer face of its leading end portion to the inner face of the leading end portion of the outer protective tube. In short, this protective tube is fixed to the metal shell such that its inner protective tube is fixed at its leading end portion to the outer protective tube and such that the outer protective tube is fixed at its trailing end portion to the metal shell.
In case this gas sensor is subjected to high vibrations for a long term, the welded portion between the inner protective tube and the outer protective tube may be broken. Then, the inner protective tube will seriously vibrate, when used, in the outer protective tube. Then, an abnormal noise is generated, or the gas sensor element may be broken, as the case may be. Moreover, the inner protective tube may move to exert adverse affects on the gas sensibility or responsibility of the sensor. In short, it cannot be said that the protective tube, especially its inner protective tube has satisfactory vibration resistance.